Many modern work machines employ hydraulically actuated work implement systems. In a typical design, one or more work implements are coupled to a linkage having actuators operable to extend, retract, tilt or otherwise move the linkage as commanded by an operator or electronic controller. In the case of a backhoe, excavator or similar work machine, the linkage includes a set of actuators for independently moving or controlling the separate linkage components.
Such work machines typically include a hydraulic system having a hydraulic pump powered by an internal combustion engine or other power source and a plurality of hydraulic actuators. Fluid pressurized by the pump may be delivered to or evacuated from the respective actuators to manipulate the linkage components. A backhoe, for example, will typically include three main linkage components, a boom, a stick and a bucket, each having at least one hydraulic actuator fluidly coupled with an implement pump of the work machine. While the respective actuators are separately controlled, it is typical for a common mechanical, pilot or electronically actuated valve to affect fluid flow to more than one actuator in the system. Therefore, the behavior of one actuator can affect fluid flow to another actuator or another part of the hydraulic system.
For example, when one of the actuators reaches an extended or retracted end of its range of motion, a relatively sudden interruption in fluid flow to or from the respective actuator can cause a pressure spike that travels elsewhere in the system. In other words, as an actuator reaches an end of its stroke, fluid flow to or from the actuator can suddenly halt as the actuator piston contacts the end of its cylinder housing. This sudden cessation of fluid flow can affect other components of the hydraulic system, similar to the “water hammer” effect common in plumbing systems, causing lurching of the work machine linkage or the work machine body itself. When a work machine is used for tasks such as positioning loads on a truck or grading soil or rock with the implement system, such a disruption can compromise the operation. The more delicate the task, the less tolerance there is for unpredictability in operation.
Operators will often attempt to avoid end of stroke effects while operating a work machine by easing off of the linkage controls prior to the linkage actuator reaching an end of its stroke. While some problems can be avoided by such manual control, the range of motion of the linkage is thereby limited, as is the efficiency of the operation. Even highly skilled operators are typically incapable of maximizing efficiency and range of motion in such instances.
Design engineers have dealt with the aforementioned problems in a variety of ways. One known method of addressing end of stroke problems is through the use of a spring or other cushion positioned internally in a hydraulic cylinder. In such a design, the hydraulic cylinder rod encounters a resistance as it approaches its end of stroke position and slows accordingly, attenuating the production of pressure spikes that may cause undesirable motion from actuators elsewhere in the system. One drawback to this approach is that there are instances where it is desirable to abruptly stop the hydraulic actuator at the end of its stroke, for example, where a sudden jarring is desirable to knock work material off of the bucket of a work machine linkage. Where a spring or similar cushion is used, the operator is less able to perform such an operation.
Other systems have been developed wherein hydraulic fluid flow to an actuator is slowed in response to the actuator approaching its end of stroke position. One such system is disclosed in U.S. Pat. No. 5,511,458 to Kamata et al. (“Kamata”). Kamata discloses an automatic cushioning control apparatus for a cylinder of a work machine that purportedly reduces shaking of the vehicle. The apparatus includes means for detecting travel direction and position of a cylinder, and for computing lever gain with respect to a signal from a control lever. A computer subsequently controls driving of the cylinder in accordance with these factors.
While Kamata and similar designs may offer improvements in performance and operator comfort in some instances, they are not without drawbacks. In particular, Kamata controls fluid flow to only one actuator of the system, and is thus unable to address effects elsewhere in the work machine hydraulic system when the actuator of interest reaches or approaches an end of its stroke.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.